Hošperk Castle, also known as Haasberg Castle or Haasber Manor (Slovene: dvorec Hošperk or German: Schloss Haasberg), is a ruined baroque style mansion located near the village of Planina, in the Municipality of Postojna, southwestern Slovenia. Originally established as a medieval fortress, the manor underwent several transformations over centuries and played a notable role in the regional history of Carniola. Once a seat of aristocratic families such as the Eggenbergs, Cobenzls, and Windisch-Grätz, it was largely destroyed during the Second World War. The site is now recognized as a cultural monument and subject to conservation efforts.

The manor stands at the south western edge of the Planina Karst field, near the entrance to Planina Cave and the intermittent Unica River, a unique hydrological feature of the region. Its position atop a gentle elevation gave it both strategic advantage and scenic prominence. Historically, it stood along key routes connecting Carniola to Trieste, which made it important both militarily and economically.

The earliest fortification on the site was known as castrum Moevntz or Unec Castle, which likely originated in the 11th century. Situated strategically along key trade and pilgrimage routes across the Karst plateau and Inner Carniola, the castle belonged to the Counts of Gorizia. It served a defensive and administrative function through the High Middle Ages.

In 1511, a powerful earthquake devastated much of western Slovenia and severely damaged the original structure. The ruins were eventually abandoned, and the feudal seat was relocated to lower ground near the Unica River floodplain.

In the aftermath of the castle's decline, the Eggenberg family, a princely house with extensive holdings in Inner Austria and the Habsburg lands, initiated the construction of a Renaissance-style manor house at the base of the old castle hill around 1614.

This early version of the manor was relatively modest—a two-story rectangular building accompanied by stables and utilitarian outbuildings. It served as an estate seat for the surrounding agricultural lands and was already equipped with elements of formality, including a small garden and a chapel.

The major transformation of Haasberg Manor began after 1716, when Princess Maria Charlotte of Eggenberg sold the estate—along with the lordships of Štemberk and Logatec—to Count Johann Caspar von Cobenzl (1664–1742). The Cobenzl family was a distinguished Austrian noble family originally from Carinthia, with substantial holdings in what is today western Slovenia and parts of the historical County of Gorizia and Gradisca, now in northeastern Italy. Johann Caspar was a loyal servant of emperor Charles VI and laid the basis for the flowering of the Cobenzl family in the eighteenth century, when three generations of Cobenzls occupied positions of influence at the imperial Court and in public affairs.

Count Johann Caspar commissioned Carlo Martinuzzi, a Venetian-trained architect active throughout Central Europe, to convert the existing residence into a grand Baroque palace. Under Martinuzzi’s direction, Haasberg was rebuilt as an elegant four-story structure, distinguished by a monumental central portal, high mansard roofs, and a balustraded balcony above the entrance.